Chemical vapor infiltration and deposition (CVI/CVD) is a well known process for depositing a binding matrix within a porous structure. The term “chemical vapor deposition” (CVD) generally implies deposition of a surface coating, but the term is also used to refer to infiltration and deposition of a matrix within a porous structure. As used herein, the term CVI/CVD is intended to refer to infiltration and deposition of a matrix within a porous part or structure. The technique is particularly suitable for fabricating high temperature structural composites by depositing a carbonaceous or ceramic matrix within a carbonaceous or ceramic porous structure resulting in very useful structures such as carbon/carbon aircraft brake disks, and ceramic combustor or turbine components. The generally known CVI/CVD processes may be classified into four general categories: isothermal, thermal gradient, pressure gradient, and pulsed flow. See W. V. Kotlensky, Deposition of Pyrolytic Carbon in Porous Solids, 8 Chemistry and Physics of Carbon, 173, 190-203 (1973); W. J. Lackey, Review, Status, and Future of the Chemical Vapor Infiltration Process for Fabrication of Fiber-Reinforced Ceramic Composites, Ceram. Eng. Sci. Proc. 10[7-8]577, 577-81 (1989) (W. J. Lackey refers to the pressure gradient process as “isothermal forced flow”).
In an isothermal CVI/CVD process, a reactant gas passes around a heated porous structure at absolute pressures as low as a few torr. The gas diffuses into the porous structure driven by concentration gradients and cracks to deposit a binding matrix. This process is also known as “conventional” CVI/CVD. The porous structure is heated to a more or less uniform temperature, hence the term “isothermal”.
In a thermal gradient CVI/CVD process, a porous structure is heated in a manner that generates steep thermal gradients that induce deposition in a desired portion of the porous structure. The thermal gradients may be induced by heating only one surface of a porous structure, for example by placing a porous structure surface against a susceptor wall, and may be enhanced by cooling an opposing surface, for example by placing the opposing surface of the porous structure against a liquid cooled wall. Deposition of the binding matrix progresses from the hot surface to the cold surface.
In a pressure gradient CVI/CVD process, the reactant gas is forced to flow through the porous structure by inducing a pressure gradient from one surface of the porous structure to the opposing surface of the porous structure. Flow rate of the reactant gas is greatly increased relative to the isothermal and thermal gradient processes which results in increased deposition rate of the binding matrix. This process is also known as “forced-flow” CVI/CVD.
Finally, pulsed flow involves rapidly and cyclically filling and evacuating a chamber containing the heated porous structure with the reactant gas. The cyclical action forces the reactant gas to infiltrate the porous structure and also forces removal of the cracked reactant gas by-products from the porous structure.
The purpose of a CVI/CVD process as described herein is to deposit a binding matrix within a porous structure. This process adds mass to the parts, and increases part density. During a CVI/CVD process, there is a correlation between the increase in part weight, and the increase in part density. Under current state of the art, CVI/CVD run times are predetermined, based on the starting densities of the parts, and the anticipated time necessary to process them to a predetermined higher density. However, CVI/CVD runs are never exactly the same due to many variables, and likewise the results of the process vary from run to run. This means that parts may be either too dense, or not dense enough at the predetermined conclusion of a run.
In order to optimize furnace processing time and increase the yield of each run, a method is desired by which the weight change of the parts can be measured during the CVI/CVD process. If the weight change of the parts during the process is known, process parameters such as furnace temperature, reactant gas flow rate, internal furnace pressure and reactant gas reactivity, may be continuously adjusted to increase, decrease, or maintain the densification rate.